linux-kernel - Re: [PATCH] sched/fair: fix calc_cfs

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Message-ID: <CAPM31R+ZkKQn1Sej8Q1kvEuLhcp-bFn7CVTKQ2ZrxfXGjC-tjw@mail.gmail.com>
Date:   Mon, 19 Dec 2016 15:32:15 -0800
From:   Paul Turner <pjt@...nel.org>
To:     Samuel Thibault <samuel.thibault@...-lyon.org>,
        Paul Turner <pjt@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

On Mon, Dec 19, 2016 at 3:29 PM, Samuel Thibault
<samuel.thibault@...-lyon.org> wrote:
> Paul Turner, on Mon 19 Dec 2016 15:26:19 -0800, wrote:
>> >> > -       if (shares < MIN_SHARES)
>> >> > -               shares = MIN_SHARES;
>> > ...
>> >> >         return shares;
>> >
>> > This will only make sure that the returned shares is 2, not 2048.
>>
>> This is intentional.  The MIN_SHARES you are seeing here is overloaded.
>> Every "1" unit of share is "SCHED_LOAD_RESOLUTION" bits internally.
>
> I'm not talking about the SCHED_LOAD_RESOLUTION scaling, but about the
> SCHED_FIXEDPOINT_SHIFT scaling, which is what
> 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
> modified on 64bit platforms.

.... From that commit:

"""
-#if 0 /* BITS_PER_LONG > 32 -- currently broken: it increases power
usage under light load  */
+#ifdef CONFIG_64BIT
 # define SCHED_LOAD_RESOLUTION 10
 # define scale_load(w)         ((w) << SCHED_LOAD_RESOLUTION)
 # define scale_load_down(w)    ((w) >> SCHED_LOAD_RESOLUTION)
"""


Please take a deeper look at the scale_load() interactions.